1. Field of the Invention
The present invention relates generally to electronic ballast controls for gas discharge lamps, and relates more particularly to an electronic ballast control with circuitry to obtain multi-function feedback sense features.
2. Description of Related Art
Electronic ballasts for controlling fluorescent lamps typically use electronics for preheating the lamp filaments, striking or igniting the lamp, driving the lamp to a given power, detecting lamp fault conditions, and safely deactivating the circuit.
Electronic ballasts for gas discharge circuits have come into widespread use because of the availability of power MOSFET switching devices and insulated gate bipolar transistors (IGBTs) that can replace previously used power bipolar switching devices. A number of integrated circuits (ICs) have been devised for driving gates of power MOSFETs or IGBTs in electronic ballasts. Examples include the IR2155, IR2157, IR2159, IR21571 and IR2167 products sold by International Rectifier Corporation and described in U.S. Pat. Nos. 5,545,955 and 6,211,623, the disclosures of which are incorporated herein by reference in their entireties.
The IR2155 gate driver IC offers significant advantages over prior circuits: The driver is packaged in a conventional DIP or SOIC package. The package contains internal level shifting circuitry, under voltage lockout circuitry, deadtime delay circuitry, and additional logic circuitry and inputs so that the driver can self-oscillate at a frequency determined by external resistors and capacitors.
The IR2157 and IR21571 products provide fully integrated ballast control ICs with several features not available in the IR2155. The IR2157 and IR21571 products function in five basic modes of operation and can make transitions between modes based on IC inputs. The modes include undervoltage lockout (UVLO) mode, preheat mode, ignition ramp, run mode, and fault mode. Other features of these ICs include: (i) a start-up procedure that insures a flash-free start without an initial high voltage pulse across the lamp; (ii) non-zero voltage switching protection circuitry; (iii) over-temperature shutdown circuitry; (iv) DC bus and AC on/off control circuitry; and (v) near or below resonance detection circuitry.
Previously available ballast ICs require external components for power factor correction (PFC) control. An example of a PFC control circuit is described in U.S. Pat. No. 6,259,614 to International Rectifier Corporation, the disclosure of which is incorporated herein by reference in its entirety.
A design challenge in providing electronic ballasts is presented by the “end-of-life detection problem,” which arises when a lamp approaches the end of its life. The IR2157, IR2167 and 21571 products described above each have a shutdown (SD) pin, used to shutdown the oscillator, pull gate driver outputs low, and put the IC in an interim micropower state. Input voltage on the SD pin above a threshold indicates lamp fault, lamp exchange, or lamp removal. It would be advantageous to provide circuitry for easier detection of the end of life of a lamp.
To address the end of life detection problem described above, one solution provides a detection of voltage across the lamp to indicate when the lamp is nearing the end of its life. The drive circuitry is disabled when the lamp voltage reaches a given threshold to avoid providing further drive signals to the lamp to prevent any damage to the power switching devices. An upper and lower threshold may be checked to determine if the lamp is operating within the threshold window for proper operation, and determine an end of life condition when the measured voltage is outside of the threshold window. An end of life detection circuit is described in U.S. Pat. No. 6,617,805 to International Rectifier Corporation, the entire disclosure of which is hereby incorporated by reference. The same lamp voltage as that used to determine end of life may be used to determine a lamp fault or lamp removal condition by comparing the measured voltage against another threshold that can be used to determine lamp presence or a lamp fault.
Typically, the voltage across the lamp is measured with a connection that is more or less directly related to the voltage applied to the lamp. Other feedback sensors are also available, such as the detection of current output through the power switching components. The current through the power switching components may be measured as a voltage across a resistor, for example, and various faults can be detected through measurement of the current, such as overcurrent, failure to strike or hard switching. If a fault is detected that indicates a malfunction in the lamp, the electronic ballast control can be reset when a lamp replacement is detected, such as through the voltage measurement across the lamp discussed above.
It would be desirable to consolidate the functions of the various fault detections and resets concerning both the electronic ballast, e.g., the switching half bridge, and the lamp driven by the electronic ballast. It would also be desirable to improve the ability and capacity of the end of life detection, while improving the number of faults capable of detection and responsiveness to those faults.
A current sense that detects current in the power switching components can usually be used to infer when an upper filament is open, or if an upper cathode is broken. The sense circuit detects overcurrent or hard switching in the power switching components, and provides a fault determination for both upper and lower filaments and cathodes. However, in the case of low voltage lamp operation or voltage mode preheat configuration, the electronic ballast may go through preheat and ignition modes even in the case of an open filament in the upper cathode without causing an overcurrent fault. Accordingly, it would be desirable to detect an open filament or broken cathode in the upper or lower portions of the lamp to handle fault conditions in a number of electronic ballast modes.